This disclosure is directed to a pilot operated safety relief valve particularly adapted for handling steam. The safety relief valve of this disclosure in conjunction with the pilot valve provided therefor is particularly useful with steam. The three phases of water become important in the operation of the safety relief valve. The ordinary circumstance requires that the valve handle steam. There is some possibility that water will accumulate in the safety relief valve or the pilot valve as steam condenses. The condensed steam is permitted to trickle along passages of the pilot valve or through the safety relief valve. This is particularly helpful in that steam is ordinarily flowed through two or three passages in both valves and any condensate which forms on the inner surfaces of either valve is permitted to trickle along the passages where the steam flows.
Condensate handling is particularly important in the pilot valve. The pilot valve is typically small in scale, made with small passages, and is often located at the extreme end of equipment, and often exposed to freezing weather. In fact, the formation of ice in the pilot valve is a real possibility should the temperature on the exterior drop sufficiently. The pilot valve is constructed and arranged to receive steam as the operative fluid. The heat from the steam actually liberated at the pilot is somewhat small inasmuch as the rate of steam flow (and related heat transfer) through the pilot valve is relatively small. The heating of the pilot valve is, therefore, minimal and chilling by the outside air is a real possibility. This is a variable dependent on the circumstances of installation, including exposure to inclement weather, prevailing winds and other factors.
The pilot operated safety relief valve is not immune to inclement environmental conditions, namely chilling from inclement weather which leads to the formation of condensate. In ordinary applications, the safety relief valve is practically closed. In the closed condition, the flow of steam through the valve is markedly reduced. This exposes it to greater formation of condensate. The apparatus is constructed with an isolated dome area. This area is particularly exposed to the risk of condensation because it is located at the end of a long flow path with little flow. The system, therefore, incorporates passages constructed to bleed condensate from the dome area back into the main line. This arrangement enables gravity flow of condensate from the dome into the main line. This condensate flow reflects a loss of steam volume and tends to increase the rate of flow of steam to the dome, and thereby raises the heat content delivered to the pilot valve and to the safety relief valve. A balance is achieved dependent on prevailing temperatures, the steam flow rate, and other scale factors.
Occasionally, it will be necessary to shut down the equipment. On the loss of steam for any reason, the equipment of this disclosure is exposed to the risk of freezing. Assuming that both the pilot valve and the safety relief valve are shut down in freezing weather, the condensate will freeze if the shut down persists for an adequate time to permit freezing. The apparatus is constructed and arranged to tolerate freezing. For instance, in the safety relief valve, freezing steam in the dome area may form a plug of ice. Even so, the ice is located so that it does not expand to thereby damage the valve structure. Should there be ice accumulated in either, the introduction of steam simply melts the ice and the problem is self-correcting.
While these and other features of the valve system of the present invention have been discussed, they will be more readily apparent from a consideration of the preferred embodiment which is summarized as a pilot operated, steam relief valve and associated pilot valve operating together. The system is particularly adapted to operate with steam. The safety relief valve incorporates a piston which closes against a valve seat, being driven by differential areas, one area being exposed to line pressure, and a larger area being exposed to pressure in a dome. The dome is connected by means of a control line to the pilot valve which is also provided with line pressure as an input. The pilot valve incorporates multiple diaphragms, one being a pressure sensing diaphragm, and the other being a boost diaphragm which assists the pilot in snap action to open. Further, the pilot valve is able to controllably respond to pressure change by means of an adjustable orifice, and also includes a second orifice which is fixed. The pilot diaphragm sensed response is a function of the control spring working against the sense diaphragm and the setting of the adjustable orifice. Other details, advantages and features of the apparatus will be understood on review of the detailed specification found below.